The "workspace" of the arm can be defined as the volume of space into which the arm can move. The primary goal of this project is to develop a device (a "linear constraining guide") for measuring force generating properties of the arm throughout the workspace and to use the device to understand underlying mechanisms of impaired arm movements in spastic hemiplegia. Although the primary goal is to use the device to elucidate mechanisms of neuromotor impairment, the proposed studies will also serve as a preliminary feasibility test of using a linear constraining guide as an assistive device for enhancing the reaching capability of a weak or spastic arm, and as a therapeutic device for providing mechanical assistance during reaching exercises in order to facilitate neuromotor recovery. The proposed experiments will focus on the patterns and causes of abnormal force generation during passive and active movements across the workspace, and specifically on the role of passive properties, reflex hypersensitivity, and abnormal synergetic muscle co-activation. To identify the contribution of each, within subject comparisons of the spastic and contralateral limb will be used in three experimental protocols. Passive properties will be measured using slow ramp stretches of the arm across a set of linear paths through the workspace. Reflex hypersensitivity will be measured using fast ramp stretches across the same set of paths. Abnormal synergetic co- activation will be measured during voluntary movements against a linear constraint provided by the guide along the same set of paths. Together, these studies will elucidate underlying mechanisms of impaired force-generating capacity in a volume of the spastic workspace.